1. Technical Field
This invention relates to impact matrix line printers in general, and more particularly, to methods and apparatus for preventing overheating of line printer hammer bank coils.
2. Related Art
Line impact matrix printers, or “line printers,” produce letters and graphics in the form of a matrix of dots by employing a “shuttle” mechanism that runs back and forth in a horizontal direction over a page of a print medium, such as single sheet or continuous form paper, coupled with the intermittent movement of the page perpendicular to that of the shuttle. An inked “ribbon” is typically interposed between the shuttle and the page. The shuttle comprises a “hammer bank,” i.e., an inline row of “hammers,” i.e., cantilevered, magnetically retracted printing tips respectively disposed at the ends of elongated spring fingers, each of which is selectively “triggered,” i.e., electromagnetically released, and timed so as to impact the page through the ink ribbon and thereby place a dot of ink on the page at a selected position. As a result of the ability to precisely overlap the ink dots produced thereby, i.e., both vertically and horizontally, line printers can produce vertical, horizontal and diagonal lines that have a virtually solid appearance, print that closely resembles that of “solid font” printers, and refined graphics similar to those produced by graphics plotters, at speeds of up to 2000 lines per minute (LPM).
Each of the hammers of a line printer hammer bank is electromagnetically actuated at least in part by the application of a current to at least one electrical coil associated with the hammer. These coils typically comprise a long strand of an electrically conductive wire (e.g., copper) that is coated with an insulator and then wound about a spool or bobbin. During printing, the sequential application of an electrical current to the coil causes it to heat up resistively, and hence, its temperature to rise incrementally. If the temperature of the coil is allowed to rise to a critical point at which, e.g., the coat of insulation on the wire is compromised, a short could occur in the coil, causing a malfunction of the associated hammer.
During the great majority of print jobs, the thermal design of the hammer bank is such that the temperature of each of the coils remains well below the critical point. However, in a relatively small number of print job types in which a highly dense pattern is printed on only a small number of adjacent hammers (so that the overall printed dot count remains about average), such as printing an uninterrupted thick black vertical line on only one or two adjacent hammers in a relatively hot room, it is possible that the temperature of the effected coils could rise beyond a desirable level.
It is known that slowing the printing speed of a line printer, i.e., skipping printing during one or more of the left/right traverses, or “strokes,” of the hammer bank, enables the coils of the hammer bank, particularly those that are cooled by ambient air, to cool down. Thus, a need exists in the relevant industry for simple, efficient systems for preventing the overheating of the coils of a line printer hammer bank that selectively reduce the speed of the printer in a “smart,” i.e., efficient, manner during the infrequent “boundary” cases described above so as to enable the coils to cool down and thereby prevent the temperature of the coils from rising to the critical temperature, but which have no effect on performance of the printer during most typical print jobs.